Microfluidic devices have been and continue to be developed for use in a number of fields. For instance, microfluidic devices are being developed for use in medical diagnostics, in which a volume of sample from a patient (such as a droplet of blood) is processed within a microfluidic device. The sample and/or other small volumes of fluids containing analytes are moved from reservoirs or other receiving chambers through microchannels to one or more reaction or association chambers to determine whether the sample contains one or more target molecules of interest (such as DNA from a pathogen). Such devices also can be configured for use in sampling air to determine the presence of pathogens or poisons by drawing in a sample of air and processing this fluid sample to identify whether DNA or another signature of interest (such as proteins uniquely associated with the pathogen) is present.
Other microfluidic devices and fields include: sorters or purifiers, in which individual cells or molecules of interest are separated from other cells or molecules by size, type, or other criteria; electrophoretic sorters, wherein different materials are separated from one another using electrophoretic force [e.g., see J. W. Parce, U.S. Pat. No. 6,337,740 (2002)]; oligonucleotide arrays, where fluids containing labeled target oligonucleotides are moved to a surface of a substrate to which complementary probe oligonucleotides are attached; protein or cell arrays, where fluids containing labeled proteins or cells are moved to a surface of a substrate to which probe proteins or cells are attached and with which the targets of interest associate; a chromatograph, in which liquid chromatography is performed [e.g., see J. M. Ramsey, U.S. Pat. No. 6,342,142 (2002)]; microfluidic printing, in which inks are formed by moving precursors through microchannels [e.g., see L. A. Kaszczuk et al., U.S. Pat. No. 6,334,676 (2002]; microfluidic mixers, in which one or more fluids are moved through a mixer inserted in a microchannel; high throughput screening devices, in which libraries of compounds are delivered to a microfluidic device which uses the compounds to determine their effects on various chemical or biological systems [e.g., see J. W. Parce et al., U.S. Pat. No. 6,306,659]; and optical systems, in which a bubble or slug of fluid immiscible in a second fluid is moved through the second fluid to a spot of optical activity on the substrate.
Each of these systems has a common requirement: moving small amounts of one or more fluids through very small channels, where surface tension of the fluid being moved is a predominant force in determining how well the fluid moves. Microfluidic systems typically analyze or process very small samples, so little sample needs to be obtained and prepared for use in a microfluidic system. However, the miniaturization of channels introduces problems of fluid movement where capillary forces predominate.
Systems have been configured in various ways to move fluids through small channels where capillary forces dominate over gravity. One configuration for moving a fluid in microchannels involves establishing a pressure differential between a point where the fluid is and a point where the fluid is to be moved. A reservoir of the fluid may be pressurized to force liquid through a microchannel to its destination, and/or a vacuum or low-pressure region may be established in the destination to draw the fluid to its destination. In microfluidic channels such as capillaries, however, large pressure differentials are needed to overcome the large flow resistance encountered in channels of such small cross-sectional area.
Other configurations have been devised to move fluids through microchannels. Such fluid pumps have been configured to utilize electrical, electrokinetic, thermal, or other driving forces to move fluids through microchannels. For example, a fluid pump may be configured to utilize an electrical driving force by configuring the electrodes and selecting the fluids so that the fluids move by electrocapillarity, electrowetting, or continuous electrowetting. A fluid pump may instead be configured to utilize an electrokinetic force such as electrophoresis, electroosmosis. A fluid pump may also be configured to utilize driving forces such as dielectrophoresis, electro-hydrodynamic pumping, or magneto-hydrodynamic pumping, by configuring the electrodes and selecting and placing the fluids within the microchannel in an appropriate manner.
Fluid pumps configured to move fluid using electrical, electrokinetic, or thermal driving forces cannot be implemented in many instances. Fluid movement depends in large part upon fluid properties, and the properties of certain fluid systems are not well-suited to use electrical, electrokinetic, or thermal forces to move the fluid.
Thus, it would be of great advantage to the field of microfluidics to provide a new means to move fluids within microchannels as are found in microfluidic devices.